I do not believe that you will find any WSM-specific grates at Ace...you will have to substitute.
The 18.5" WSM top grate is 17.5" in diameter.
The replacement grates for the 18.5" kettle will work in the top position....choose basic or flip up.
That same grate (17.5") will work in the lower position but it will rest on the bolts.
This puts the grate about 1-1/2" higher than the OEM position.
The actual lower grate diameter needs to be 17" to rest in the OEM position.
The 22.5 kettle charcoal grate is 17" but it is not nickel plated.
The 18.5 WSM charcoal grate diameter is 15.5"
The 18.5 kettle charcoal grate is 13.5" and can rest on your OEM charcoal grate and inside of your 14.5" coal ring.
Clock them 90 degrees apart.
My Home Depot's grate prices are cheaper than Ace.